Spiral antennas are known for obtaining a wide bandwidth in transmitting or receiving electromagnetic radiation. See, for example, U.S. Pat. No. 2,977,594. A spiral antenna generally comprises two conductors, spaced from each other and interwound in a planar spiral. A spiral antenna may be energized at its center by means of a cable with one conductor of the cable connected to one conductor of the spiral and another conductor of the cable connected to the second conductor of the spiral. A spiral antenna has an operating bandwidth determined by the circumference of the conductors which form the spiral. Generally, each curved section within the spiral corresponds to a different wavelength within the bandwidth. A section of the spiral becomes activated where currents in the two conductors at a given frequency are substantially in-phase. The lower frequency limit of the antenna is determined by the outermost or largest diameter of the spiral, and the upper frequency limit is determined by the diameter of the spiral where the conductors are at their smallest dimension that still contains a spiral curvature at the center of the antenna. Thus, a spiral antenna may transmit or receive a broad bandwidth of frequencies within these two geometrically determined limits.
When such a spiral antenna is energized by radio frequency energy, it radiates a broad, circularly polarized unidirectional beam from each side of the plane of the spiral. Each radiated beam is normal to the plane of the spiral.
With spiral antennas, it must be remembered that the direction (clockwise or counter clockwise) of the spiral depends on which direction the spiral is being viewed from. The term "configuration sense" is used to indicate the direction of rotation as one proceeds outward from the center of the spiral as viewed from one side. Thus a single antenna element actually has two configuration senses depending on which side is viewed. When a spiral antenna is energized, the polarization of the beam on any one side corresponds to the configuration sense of the spiral as viewed from the opposite side. Accordingly, the two radiated beams are identical except that the polarization of the radiated field on one side is the opposite of that on the other.
Generally, in the art of antennas, a transmitted beam is characterized as polarized either horizontally or vertically with respect to the ground. With conventional straight-wire antennas, a receiving antenna can receive a signal from a transmitting antenna only to the extent the two antennas share the same horizontal or vertical polarization. For example, theoretically, a wire antenna perfectly perpendicular to the ground would not be able to receive a signal from a transmitting antenna which was parallel to the ground. An analogous principle applies to spiral antennas. With a spiral antenna, a transmitted beam has a property of either left or right hand circular polarization. A right-hand-polarized side of a spiral antenna can receive signals only from a right-hand-polarized side of a transmitting antenna. In most real-world situations, it is necessary for an antenna on board a missile or aircraft to receive a signal regardless of circular polarization. One way of receiving signals of either polarization is to employ two spiral antennas, with opposite configuration senses. See, for example, U.S. Pat. No. 2,977,594, FIG. 2. With two antennas, the portion of a signal that cannot be received by one antenna will be received by the other antenna. It is therefore desirable to provide a geometric arrangement of spiral antennas having different configuration senses, and therefore able to receive signals regardless of circular polarization, which occupies the space normally occupied by a single spiral antenna.
A problem with known spiral antenna geometries is that it is difficult to employ multiple antennas to form a broadband antenna system and still fit within the space normally occupied by one of the antennas alone. For example, radar warning receivers in missiles and aircraft generally utilize a spiral antenna housed in a space provided in the frame of the aircraft. Known antenna systems covering the required bandwidth for this situation generally comprise multiple spiral antennas which will not fit in the space provided for an existing single spiral antenna. To install a multiple spiral antenna system, it is necessary either to modify the frame of the aircraft by increasing the amount of space provided for a single antenna or to deform the geometric pattern of each antenna to fit the available space. Modification to the aircraft frame is undesirable because it is costly and time consuming. U.S. Pat. No. 4,559,539, assigned to the assignee of the present invention, is one example of a group of spiral antennas within a circular housing. The antenna disclosed in that patent, however, is not a dual polarization configuration sense antenna but, rather, a group of single polarization configuration sense antennas with different bandwidths.
The present invention provides a novel, non-obvious, solution to the problem of fitting a plurality of spiral antennas having different configuration senses into the space of a single spiral.